How much air pressure am I losing when detaching my bike tire pump?
QUESTION: After pumping up my bike tires, I always hear the sound of air loss when disconnecting the hose. How much above the pressure I want should I pump the tires to compensate for the loss? I have Presta valves. —Darryl J.
RBR’S STAN PURDUM REPLIES: In theory, at least, and generally in actual practice, you aren’t losing much air pressure at all. The sound you hear is the pressure that’s built up in the pump escaping into the atmosphere.
When you first attach the chuck of your pump to the valve stem, the space in the tube (or tire, if you’re running tubeless tires) and the space in the pump becomes a single closed system. If there’s already some air pressure in the tube, some of that pressure moves into the pump, equalizing the pressure in the entire system. This is what causes the gauge on your pump to register the current psi. Then, as you operate the pump, the pressure in the entire system increases. When the gauge tells you you’ve reached the desired psi, you carefully remove the chuck, and the pressure in the tube causes the Presta valve to close almost instantly. The pfff you hear is the pressure in the pump escaping. Then, assuming you don’t accidentally depress the valve plunger, either while removing the chuck or while tightening down the captive knurl nut, the pressure in the tube remains at about the psi you wanted.
None of this is particularly hard to do with a floor pump, but in my experience, Presta valves can be fiddlier when using a frame pump, particularly if it’s the sort where the chuck has to be screwed to the threads of the valve’s outer wall to achieve a seal. Almost invariably, I manage to lose some pressure from the tube while unscrewing the chuck after pumping air in. A frame pump that seals to the valve by either a lever or a push-on diaphragm is less likely to cause a pressure loss as you detach it.
Back to your question, however, since even when everything goes well with whatever kind of chuck you have on your pump, it’s possible to lose a tiny amount of pressure while the valve plunger is closing. So I usually pump to a slightly higher psi than I want (e.g., to 82 psi if I want to end with 80). There’s probably no science behind exactly how much beyond my target psi I should go, but adding a little extra makes me feel better.
Stan Purdum has ridden several long-distance bike trips, including an across-America ride recounted in his book Roll Around Heaven All Day, and a trek on U.S. 62, from Niagara Falls, New York, to El Paso, Texas, the subject of his book Playing in Traffic. Stan, a freelance writer and editor, lives in Ohio. See more at www.StanPurdum.com.
Stan, you described how Schrader valves work.
Presta valves don’t release any pressure when the pump chuck is properly attached. The air pressure from the first pump stroke opens the valve when the pressure exceeds the air pressure in the tire.
When the chuck is properly removed the amount of air lost is insignificant. The pressure in the tire closes the valve as the pressure in the pump hose decreases.
I see your point, and your explanation of the function between the pump and tube when using a Presta valve is better than mine. I appreciated your clarity. I probably should have explained that the pump and tube don’t become a single closed system until the the first stroke of the pump. But with either type of valve, the sound of escaping air is from what remains in the pump at time of detachment, not from pressure loss in the tube.. With Presta valves, however, when using a frame pump, my fumble fingers sometimes manage to depress the valve while detaching the pump.
With Presta valves, if one fills them often, it becomes easy to tell if the air released when disconnecting the chuck is just the air in the pump hose or also includes air from the tube when the valve plunger is accidently hit. One can usually tell by the amount of air released.
Depending on how the presta head attaches… I use the Silca method. Once the target pressure is reached, put a pair of forked fingers around the valve stem and push straight down quickly. You’ll hear a rather loud pop when the air leaves the pump hose, but likely almost no air leaves the tire. “Josh” on the Silca site shows this with the “big” push-on presta head, but I also use it on their (expesive) Hiro head.
When I am pumping with my floor pump (Silca push-on chuck) i can hear the Presta valves click with each pump stroke. This clearly indicates that the valve closes after each pump stroke. Unless I flub removing the chuck (hard to do since it’s a straight pull-off), I lose zero psi.
Going off at a tangent, my hunch is that the built in pressure gauges on most bike pumps are inaccurate to the point that we rarely achieve our target pressure, but are close enough.
Any perceived loss of pressure on pump release is probably within the margin of error of the gauge. Just my theory, and I’d be interested to see gauge test data!
When I got new converted to tubeless with new rims and tires, I purchased a Topeak JoeBlow Booster floor pump. At first, I used it the same way as I did with my conventional pump. When I removed its pump head from the valve stem, I heard a lot of hissing as the pump dumped all the compressed air from its chamber. Then I decided to try a new method with the Booster pump. So, after pumping up my tire, I flipped the selector on the pump back to the “charge” position. Now when removing the pump head, the Presta valve closed immediately and there was just a split-second puff of air from the pump head. There was no release of air from the chamber of the pump. I now have confidence that I can detach the pump head and stay at my target air pressure.